The present invention relates to the field of fluorinated hydrocarbons and its subject is more particularly the manufacture of 1,1,1-trifluoroethane (known in the trade under the name F143a) by fluorination of 1-chloro-1,1-difluoroethane (F142b) with arhydrous hydrofluoric acid (HF).
Since chlorofluorocarbons (CFCs) were identified as one of the factors responsible for accelerating the deterioration in the stratospheric ozone layer, political and industrial players have been irrevocably committed to a process of substitution of CFCs. This substitution process relates to essential industrial sectors, such as the food refrigeration procedure, building insulation, air conditioning, microelectronics, and the like.
Searches to find replacements for these compounds were firstly focused on products containing hydrogen atoms (HCFCs) and then on products which no longer contain chlorine: hydrofluorocarbons (HFCs).
One of these HFC compounds, which does not contain chlorine and is thus without effect on the ozone layer, is 1,1,1-trifluoroethane (F143a). This compound is mainly intended, as a mixture with other HFCs, to replace F22 (chlorodifluoromethane) and F502 (azeotropic mixture of F22 and chloropentafluoroethane) in the field of refrigeration, air conditioning and other applications. There is thus great advantage in developing the simplest possible process for producing F143a.
Various processes for the preparation of F143a have been described in the literature. Thus, it is known to prepare F143a by fluorination of 1,1,1-trichloroethane, either in the gas phase (U.S. Pat. Nos. 2,744,148 and 2,744,147) or in the liquid phase. In the latter case, the fluorination is preferably carried out in the presence of a fluorination catalyst and Patent Application WO 96/5156 recommends in particular the fluorination of trichloroethane in the presence of pentavalent antimony halides.
The fluorination of vinylidene fluoride (CF2=CH2) has also been described, both in the gas phase (U.S. Pat. No. 2,669,590) and in the liquid phase (EP 703,204). This last process can be carried out in the absence of catalyst and would provide excellent results. However, on account of the high cost of vinylidene fluoride, such a process does not appear to be viable industrially; this is because vinylidene fluoride is obtained industrially by pyrolysis of 1-chloro-1,1-difluoroethane (F142b) which is itself generally obtained by liquid phase fluorination of 1,1,1-trichloroethane or of vinylidene chloride in the absence or in the presence of catalyst (see, for example, Patents EP 98341, FR 2,365,542 and EP 421,830). 
F142b is an important industrial product which is used as starting material in the manufacture of vinylidene fluoride (VF2) but which, as HCFC, is also used as a replacement for certain CFCs, in particular as blowing agent in the foam industry and as propellent in the aerosol industry.
A process for the direct and economical manufacture of F143a from F142b is thus particularly advantageous. There exist in the literature only two publications relating to the fluorination of F142b to F143a (U.S. Pat. Nos. 3,456,025 and 2,767,227). These processes consist in carrying out the fluorination of F142b in the gas phase at high temperature in the presence of a fluorination catalyst. Although these two patents do not teach anything regarding the lifetime of the catalyst, it is known that the disadvantage of gas phase fluorination processes generally lies in a rapid deactivation of the catalyst.
It has now been found that, in the presence of a fluorination catalyst, F142b and anhydrous hydrofluoric acid react very rapidly in the liquid phase to form F143a very selectively.
The subject of the present invention is thus a process for the manufacture of F143a which is easy to implement industrially, characterized in that it comprises the fluorination of F142b with anhydrous hydrofluoric acid in the liquid phase in the presence of at least one fluorination catalyst.
The fluorination catalyst(s) which can be used in the process according to the invention are active catalysts which are well known for liquid phase fluorination reactions, such as halides, oxides and oxyhalides of elements belonging to groups IIIa, IVa and Va and to subgroups IVb, Vb and VIb. Use may more especially be made, among the elements selected from the columns of the Periodic Classification, of titanium, niobium, tantalum, molybdenum, boron, tin and antimony. Compounds containing antimony are particularly well suited. Halides are more particularly chosen as antimony derivatives; a typical example is antimony pentachloride SbCl5 or the pentavalent antimony chlorofluorides formed in situ by partial fluorination of SbCl5.
The amount of catalyst to be employed in this liquid phase fluorination can vary within wide limits. Expressed as a percentage by weight of metal, in particular of antimony, the catalyst content of the liquid reaction mixture is, however, generally between 0.01 and 10%, preferably between 0.1 and 5%.
The possibility of being able to use such small amounts of catalyst is entirely surprising because liquid phase fluorinations are generally carried out in the presence of large amounts of catalyst. Thus, in the process for the preparation of F143a by fluorination of 1,1,1-trichloroethane, as described in the abovementioned Patent Application WO 96/5156, the catalyst content of the reaction mixture is from 15 to 50%.
The pressure under which the process according to the invention is carried out is not critical in itself, from the moment that it enables the reaction to be carried out in the liquid phase, that is to say that it is sufficient to maintain the reactants present in the reactor essentially in the liquid form. It varies according to the temperature of the reaction mixture and according to the composition of this reaction mixture. The absolute pressure of the reaction system is generally chosen between 5 and 30 bar, preferably between 7 and 20 bar. The fluorination reaction of F142b to F143a releases hydrochloric acid. If it is desired to separate this HCl by distillation, it is advantageous to carry out the fluorination under a pressure which is sufficiently high to be able to condense the HCl under good conditions and, in this case, it is advantageous to operate at a pressure greater than approximately 11 bar. However, this is not essential and the HCl can be separated by any means other than a distillation, such as, for example, washing with water.
The process according to the invention can be carried out within a wide temperature range. Generally, the process is carried out at a temperature greater than 0xc2x0 C. and less than 120xc2x0 C. However, the temperature advantageously does not exceed 100xc2x0 C. and the reaction is preferably carried out at a temperature of between 10 and 85xc2x0 C. The use of such low temperatures, made possible by the very high reactivity of F142b, makes it possible to minimize the formation of by-products.
For the purpose of maintaining the activity of the catalyst, in particular that of antimony pentahalides, and of preventing deactivation by reduction to antimony trihalide, it may be advantageous to carry out the fluorination in the presence of a small amount of chlorine. The chlorine can be added periodically or continuously. The amount of chlorine supplied with the F142b is generally less than 2.5 mol of chlorine per 100 mol of F142b, preferably less than 1 molar %. The amount of chlorine can be very low, indeed zero, and it becomes lower as the temperature becomes lower. This is because it has been shown that, although it is difficult to make F142b react with chlorine, in the presence of SbCl5 or of other Lewis acids, it nevertheless reacts to give, inter alia, chlorination products of the 130 series (CFCl2CH2Cl, CF2ClCH2Cl, CCl2=CHCl, and the like) and especially of the 120 series (CCl3CHCl2, CCl2FCHCl2, CF2ClCHCl2, and the like). At low temperature, these chlorination reactions are only very slow, chlorine consumption is very low and very low amounts of chlorine are sufficient to maintain the activity of the fluorination catalyst. It is thus preferable to use between 0.05 and 0.5% of chlorine (molar % with respect to F142b).
The process according to the invention can be implemented batchwise but it is advantageously carried out continuously. In the latter case, the reaction can be carried out in conventional equipment well known to the person skilled in the art. It can be a reactor supplied, in the gaseous or liquid form, with the starting materials (F142 and HF) and the recycled materials and appropriately heated or cooled. It must promote contact between the reactants by an appropriate geometry, an appropriate method of introduction of the reactants and an appropriate mixing technique. The reactor can be surmounted by a column and by a retrograde condenser which makes it possible to prevent departure, in the gas flow exiting from the reactor, of the catalyst or catalysts used and to adjust the composition of organofluorinated compounds in this flow (content of F143a, F142b, and the like).
The starting materials or recyclates are supplied to the reactor in the ratio appropriate for production of F143a. This means, in the case of a complete recycling of the unconverted products, a fresh HF/fresh F142b molar ratio in the region of stoichiometry, that is to say in the region of 1. In practice, in order to take into account the HF removed with the F143a formed, the reactor is generally supplied with an HF/F142b mixture corresponding to an HF/F142b molar ratio slightly greater than 1, generally between 1.05 and 1.20.
As indicated above, the use of chlorine for maintaining the catalytic activity is reflected by the formation of chlorination products (essentially products of the 130, 120 and 110 series) or of other by-products which are heavy products in comparison with F143a and F142b and which therefor have a tendency to accumulate in the reactor. The concentration of heavy by-products not belonging to the 140 series in the reaction mixture is not critical but it has been found that the reaction is easier to carry out when this content is not excessively high. As a general rule, it is regulated so as to remain less than 75% by weight; the content of heavy by-products in the liquid reaction mixture can be adjusted by a purge of the reaction mixture.
The flows, gaseous and optionally liquid, resulting from the reaction are treated conventionally in order to separate the useful finished products therefrom (F143a, HCl). As regards the recovery of the hydrochloric acid, this treatment can in particular comprise a distillation of anhydrous HCl or washing the gas flow with water in order to separate the HCl and the organic products (essentially F143a). The F143a, the F142b, the other products of the 140 series (F141b, F140a) possibly formed, the unconverted hydrofluoric acid and the catalyst or catalysts used contained in the liquid purge of the reaction mixture can be recycled to the reaction system.